This invention relates to copy protection to selectively prohibit the recording (i.e., copying) of analog video signals output from video discs, digital VCRs, digital broadcasting receivers, computers or other digital devices, yet permits satisfactory display by TV receivers of the original copy-protected video signals.
With the development of digital technologies, it has become easier and more commonplace to provide high-quality video signal sources for consumer use. For example, digital broadcasting terminals and DVDs (digital video discs) provide users with high-quality video signals and constitute such high-quality video sources. Digital VCRs (video cassette recorders) are gradually increasing their share of consumer use. Currently, however, analog VCRs are much more popular, and providers of high-quality video sources must take into account the coding or processing of digital video signals so as to restrict or prohibit the copying by analog VCRs of the analog version of digital video signals in order to protect the copyright of such video signals.
There are known techniques for prohibiting copying by analog VCRs known as, for example, the AGC pulse system and the color stripe system. The AGC pulse system restricts or prohibits copying of an analog video signal by inserting pulses having an amplitude larger than the normal AGC reference level typically recognized by the AGC circuits of the analog VCR. That is, pseudo sync pulses are inserted in some of the vertical blanking periods of the video signal, as shown in FIG. 21A. FIG. 21B is an enlarged view of a line interval in the blanking period in which the pseudo sync pulses are inserted. The pseudo sync pulses may comprise five cycles of a pulse of level p added to the horizontal sync pulse. Thus, the usual AGC reference level, measured as the level of the horizontal sync pulse, is increased by p.
Many types of analog VCRs (such as consumer-use analog VCRs) are configured to perform the AGC operation by using the horizontal sync pulse in a 1H line interval in the vertical blanking period, as shown in FIG. 21C. In these VCRs when the pseudo sync pulse having a larger amplitude than the horizontal sync pulse is inserted in the 1H line interval, the AGC circuit executes its AGC operation on the amplitude of the pseudo sync pulse as if the pseudo sync pulse is the reference level. As a result, the gain of the AGC circuit is erroneously thought to be too large and, accordingly, is decreased so as to be too small to detect the sync signal and to normally reproduce the video signal. TV monitors, however, use an AGC system that differs from that of analog VCRs and can adequately display reproduced images notwithstanding the pseudo sync pulses.
However, those video cassette recorders having long AGC time constants, such as .beta.-system VCRs, 8 mm VCRs and some VHS-system VCRs, are not particularly sensitive to pseudo sync pulses. To account for this, the color stripe system has been proposed. In the color stripe system, the phase of the color burst signal in the reproduced video signal is inverted for N-out-of-M lines; for example, 4 out of 21 lines.
TV monitors use APC (automatic phase control) for extracting color burst signals from the video signal (more particularly from the chroma signal). The phase of the color burst signal is detected and phase changes are integrated by a low pass filter, for example, to control a voltage controlled oscillator to generate a reference subcarrier for color demodulation of the input video signal. The reference subcarrier is also used as a reference signal for phase detecting the burst signal.
When a video signal processed by the color stripe system is recorded on another consumer-use analog VCR, the APC circuit of the analog VCR erroneously recognizes the phase-inverted color burst signal as if it is the original color burst signal. This results in inverting the color of the line when that video signal is reproduced. As a result, color-inverted lines appear in the displayed image every 21 lines, as shown in FIG. 22.
On the other hand, typical TV receivers use long time constant APC circuits having a narrow frequency response range for generating the reference subcarrier wave for color signal demodulation, so that the reproduced images are not affected when the color burst signals in only four lines are inverted. However, for those APC circuits that exhibit a long time constant but a wide frequency response range, color-inverted lines may appear in the displayed image.
In view of this, the assignee of the present invention has proposed that for all lines in an effective image display, the phase of only a certain period of the burst signal is inverted. This prevents deterioration of the quality of images even with TV monitors that exhibit color-inverted lines when using the color stripe system.
In consumer-use analog VCRs, the color subcarrier of frequency of 3.58 Mhz (in case of NTSC) is frequency-converted into a low-band-converted color signal having a center frequency in the range of 600 to 700 kHz. The low-band-converted color signal is frequency-multiplexed with a luminance signal and recorded on magnetic tape. When the video signal is reproduced, the low-band-converted color signal is frequency-separated and converted into a color signal having the original carrier frequency. Consequently, because of such recording/reproducing processing and the characteristics of the electromagnetic elements (e.g., tape, heads, etc.), the color signals in consumer-use analog VCRs are limited to a much narrower band than TV receivers. Because of this narrow band of the color signals, the duration of the reproduced color burst signal tends to expand relative to the duration of the original signal prior to recording. FIG. 23A shows the color burst signal in an original signal and FIG. 23B shows the expanded color burst duration in the signal reproduced by the VCR. The original color burst signal a shown in FIG. 23A is expanded to a' as shown in FIG. 23B when recorded on and then reproduced from the magnetic tape.
This inherent characteristic is used in the above-mentioned proposal by the assignee to invert the phase of only a few cycles of the color burst signals to prohibit copying. FIGS. 24A to 24C show an example of this proposal. For normal lines, the color burst signal extends over 10 cycles, for example (FIG. 24A). In some lines, however, in order to induce image disturbance for copy protection, 13 cycles of the color burst signal are provided, including 6 cycles of the phase-inverted color burst signal and 7 cycles of the normal phase color burst signal (FIG. 24B). For example, a block of 21 lines includes 17 lines with regular (i.e., normal phase) color burst signals and four lines with inverted color burst signals, these blocks being repeated throughout the image (FIG. 24C). When this type of copy-protected video signal is reproduced, the APC circuit in the color sync circuit of the VCR cannot follow the burst signal. As a result, the quality of the image displayed from the copied signal is deteriorated, and copying of the signal is prevented. However, TV monitors detect and use the normal phase color burst signals to display substantially normal images from the original, non-copied, copy-protected video signal.
This type of copy protection relies on VCRs that record color signals having narrow frequency bands and is not very effective in S-VHS analog VCRs, for example, in which the color subcarrier extends over a wide band. To overcome this problem, it is suggested to increase the number of cycles of inverted color burst signals in the burst interval. FIG. 25 shows an example in which the 13 cycles of burst signal include 10 cycles of inverted phase and three cycles of regular (i.e., normal) phase. This assures the copy prohibit function even with VCRs that record with wide band color signals. However, depending on the TV monitor, images displayed thereby may be unacceptably deteriorated. For example, even when no phase inversion of the burst signal occurs, images may be distorted by displaying a difference in color saturation resulting in horizontal stripes or by changes in color saturation over the entire image.
The present inventor attempted to overcome this problem of different color saturation by proposing blocks of copy prohibit signals formed of lines containing copy prohibit signals with a larger number of inverted color burst signals alternating with lines containing increased amplitude of regular color burst signals. Here the repetitive frequency of regular-phase color burst signals increases beyond the frequency response range of TV monitor APC circuits, thereby preventing or at least minimizing image disturbances caused by inverted color burst signals. Since normal phase color burst signals of increased amplitude are used, the decrease in cycles of regular color burst signals (caused by an increase in the number of cycles of inverted color burst signals) is compensated, and image disturbances on the TV monitors are prevented. On the other hand, when these copy-protected signals are copied and reproduced by VCRs, image disturbances are generated as a result of the expansion of the inverted color burst signals (FIG. 23B).
As known, TV monitors use an ACC (automatic color correction) circuit for automatic correction of color signal levels or amplitudes. The ACC circuit may use diode detection or phase detection, the output of which is integrated. When the ACC circuit uses phase detection, the above copy-protection method causes a substantial decrease in the color burst signal levels due to a cancellation of the integrated value of the detected inverted color burst signals because the inverted color burst signals are substituted for the regular color burst signals. However, since only some of the cycles of the color burst signals are modified (i.e., phase inverted), they compensate for the decreased level, and cancel the image disturbing function.
When the ACC circuit uses diode detection, phase information does not affect the detector output; for example, inverted color burst signals do not significantly affect the detector output but, rather, the inverted color burst signals result in overcorrection and cause image disturbance. A problem of ACC circuits is that their operating characteristics vary with different TV monitors so that the amount of correction that is needed is not determined definitely. Therefore, when a TV monitor is supplied with a signal containing such copy prohibit signals, it uses a fixed value to effect correction against the disturbance and residual errors may be detected as a disturbance. To overcome this problem, the inventor proposed a method of using inverted color burst signals and regular color burst signals of increased amplitude in a single horizontal line; and permitting the user to modify the amplitudes of the color burst signals as desired. If overcorrection by burst signals of increased amplitude occurs, image disturbance can be removed from TV monitor displays by appropriately setting the amplitude of such color burst signals.
While this method is effective when a video signal is supplied to a single TV monitor, it may be ineffective when the video signal is supplied to two or more monitors. For example, when two or more TV monitors are supplied from a single video output, it is difficult to effect appropriate corrections in all monitors to remove fully the disturbance in the display. This problem also occurs when a single TV monitor has a multi-screen display, such as a main screen and a sub-screen (e.g., picture-in-picture). In this case, since the sub-screen displays an image of the main screen in smaller size, the single monitor has separate circuits for processing the signals for the main screen and the sub-screen; and elements used in the respective circuits often are based on different standards. This may result in different characteristics of the ACC circuits and the APC circuits in the main screen and the sub-screen. Consequently, even after an appropriate setting is made for one, image disturbances may occur when the image displayed on the main screen is switched to the subscreen.